MECHANICAL
ENGINEERING SYSTEMS
LABORATORY
Group 02
Asst. Prof. Dr. E. İlhan KONUKSEVEN
FUNDAMENTAL
CONCEPTS IN
MEASUREMENT AND
EXPERIMENTATION
HOW TO MEASURE ?
BY MEANS OF SENSING DEVICES
OFTEN CALLED:
TRANSDUCERS
or
SENSORS
instead of using human senses
Sensors
used for both
Actuators
HUMAN SENSES
• LACKING IN LATITUDE
(SCOPE AND EXTENT)
• RELATIVE
• LIMITED
• SLOW
• BIASED
HUMAN SENSES
• Our ability to sense cold and hot is only
relative, with a selectivity of the order of 1°C
and only over a very limited temperature
range.
• Measuring a high electric voltage with
human senses is extremely hazardous.
CLASSIFICATION OF TRANSDUCERS
A transducer can be defined as a device capable of
converting energy from one form into another. Transducers
can be found both at the input as well as at the output stage
of a measuring system.
We can distinguish six different energy domains:
(1) Radiant
(2) Mechanical
(3) Thermal
(4) Electrical
(5) Magnetic
(6) Chemical.
CLASSIFICATION OF TRANSDUCERS
The input transducer is called the sensor, because it senses
the desired physical quantity and converts it into another
energy form.
CLASSIFICATION OF TRANSDUCERS
The output transducer is called the actuator, because it
converts the energy into a form to which another
independent system can react, whether it is a biological
system or a technical system.
For a biological system the actuator can be a numerical
display or a loudspeaker to which the visual or aural senses
react respectively.
For a technical system the actuator could be a recorder or a
laser, producing holes in a ceramic material. The results
can be interpreted by humans.
TRANSDUCER TYPES :
• SELFGENERATING
(Passive)
• NON-SELFGENERATING
(Active)
TRANSDUCER TYPES :
• SELFGENERATING (Passive)
A self-generating transducer is defined as a transducer
which requires no auxiliary energy source to convert
energy.
Examples of a self-generating transducer which can be
found at the input of a system are a solar cell or a
thermocouple, and at
the output we can find for instance a heating element
converting electrical energy into thermal energy without
the need of an auxiliary energy source.
TRANSDUCER TYPES :
•NON-SELFGENERATING (Active)
Requires an auxiliary energy source to convert energy
from one domain into another.
A good example of a modulating transducer which is
found at the input of a system is a strain gauge. This type
of transducer requires an electrical energy source to
become operational. The electrical current flowing in the
strain gauge is modulated by a mechanical force which is
converted into an electrical voltage change via a change
of the resistive elements.
TRANSDUCER TYPES :
Transducer
Transistor
Thermocouple
pH meter
LED display
LCD display
Coil
Magnetoresistor
Photoconductor
Type description
Modulating shape transducer
Self-generating input transducer
Self-generating input transducer
Self-generating output transducer
Modulating output transducer
Self-generating output transducer
Modulating input transducer
Modulating input transducer
A collection of electronic self-generating transducers with examples of 'shape' transducers on
the main diagonal where no energy conversion is involved.
SENSORS :
ANY OBJECT THAT HAS A PROPERTY
INFLUENCED BY A VARIABLE IS
POTENTIALLY A SENSOR WHICH MAY
BE USED TO MEASURE THAT
VARIABLE
INPUT
ENVIRONMENT
TRANSDUCER
an interaction is necessary
between Transducer and
Environment in order to
perform a measurement.
OUTPUT
MEASURAND
THE PARTICULAR SYSTEM PARAMETER
/ VARIABLE BEING OBSERVED BY A
TRANSDUCER DURING THE
MEASUREMENT PROCESS IS CALLED
THE MEASURAND
INPUT - OUTPUT CHARACTERISTICS
INPUT
SIGNAL
TRANSFERRED
(MEASURED
TO
THE
VARIABLE)
OUTPUT
IS
SIGNAL
(OBSERVED VARIABLE) BY SOME FUNCTIONAL
RELATIONSHIP (INSTRUMENTATION)
DESIRED
INPUT
FD
DESIRED
OUTPUT
UNDESIRED INPUTS
ALL
THE
INPUTS
MEASURAND
TRANSDUCER
OTHER
ACTING
ARE
THAN
ONTO
CALLED
INPUTS OR DISTURBANCE
THE
THE
UNDESIRED
UNDESIRED INPUTS :
• INTERFERING INPUTS
• MODIFYING INPUTS
INTERFERING INPUTS :
REPRESENT THE INPUTS TO
WHICH THE INSTRUMENTATION IS
UNINTENTIONALLY SENSITIVE
EXAMPLE :
THE MAGNETIC FIELD DUE TO A
NEARBY ELECTRIC MOTOR OR
TRANSFORMER
MODIFYING INPUTS :
THESE INPUTS EFFECT THE
RELATION ‘BETWEEN THE DESIRED
AND/OR INTERFERING INPUT’ AND
‘THE DESIRED OUTPUT’
EXAMPLE :
STRAIN GAGE IS A TRANSDUCER
IT IS GLUED ONTO A SPECIMEN TO MEASURE STRAIN - ITS
RESISTANCE CHANGES IF IT IS STRECHED
ITS RESISTANCE DEPENDENT ON TEMPERATURE
It changes the gage resistance due to the strain resulting from the
differential expansion of the gage and the specimen since they are made
out materials of different “thermal expansion coefficients”.
TEMPERATURE IS AN “INTERFERING INPUT”
the gage temperature is also a “modifying input” since the “gage factor”
is sensitive to the temperature.
TEMPERATURE IS ALSO AN “ MODIFYING INPUT”
DESIRED INPUT
FD
FMD
MODIFYING INPUT
FMI
INTERFERING INPUT
FD
DESIRED
OUTPUT
Functional Stages of Measuring Systems:
Most measuring systems consist of the following three stages:
Sensor (or Transducer) is always the 1st stage of every measuring system
and it produces an output to represent the measured quantity (measurand).
Intermediate Stage modifies the sensor output (transduced information) so
that it becomes acceptable to the terminal stage. All processes like
amplification, filtering, conversion, integration, data transmission,
telemetering are performed at this stage.
Terminal Stage includes all possible data storage, monitoring, processing,
displaying, plotting features.
SENSOR
Measurand
INTERMEDIATE
STAGE
Sensor
Output
(Combined single instrument: Mercury in glass thermometer)
TERMINAL
STAGE
Measurement
Data
GENERALIZED MEASUREMENT
SYSTEM
MEASURED
VARIABLE
DATA
TRANSMISSION
TRANSDUCER
DATA
MANIPULATION
DATA
DISPLAY
OBSERVED
VARIABLE
DESIGNING A MEASUREMENT
SYSTEM REQUIRES 2 THINGS :
DESIGN OF VARIOUS
INDIVIDUAL FUNCTIONAL
ELEMENTS
INTERCONNECTION OF
VARIOUS ELEMENTS
Perfect Measurement:
There exists no such thing like “Perfect Measurement” due to
variety of reasons like:
Presence of Noise (interfering effects),
The effects other than the measurand (i.e.; undesirable inputs)
may produce undesirable changes in the transducer output which
make the measurement susceptible to noise.
Transducer Dynamics,
The ideal I/O dynamics of a transducer is of with “no dynamics”,
that is with a “static characteristics” or simply with an
“instantaneous response”.
+
R1
Voltmeter
V
-
R2
Loading Effect,
Since the transfer of information between a transducer and its
environment is accompanied by an energy transfer, the presence
of a sensor may well result in some structural changes in its
environment (and vice versa) which in turn affect the state of the
measurand. This is often referred to as “Loading Effect”.
U
Rm
Measured Voltage Vm